A large number of both synthetic and naturally occurring low molecular weight ligands are known that interact with DNA via a number of different mechanisms, including covalent or non-covalent interaction in the minor or major grooves, intercalation between base pairs or other types of non-specific interactions.
A particular class of compounds which interacts with the minor groove are the pyrrolobenzodiazepines (PBDs). PBDs have the ability to recognise and bond to specific sequences of DNA; the most preferred sequence is PuGPu (Purine-Guanine-Purine). The first PBD antitumour antibiotic, anthramycin, was discovered in 1965 (Leimgruber, et al., J. Am. Chem. Soc., 87, 5793-5795 (1965); Leimgruber, et al., J. Am. Chem. Soc., 87, 5791-5793 (1965)). Since then, a number of naturally occurring PBDs have been reported, and over 10 synthetic routes have been developed to a variety of analogues (Thurston, et al., Chem. Rev. 1994, 433-465 (1994)). Family members include abbeymycin (Hochlowski et al., 1987 J. Antibiotics, 40, 145-148), chicamycin (Konishi et al., 1984 J. Antibiotics, 37, 200-206), DC-81 (Japanese Patent 58-180 487; Thurston et al., 1990, Chem. Brit., 26, 767-772; Bose et al., 1992 Tetrahedron, 48, 751-758), mazethramycin (Kunimoto et al., 1980 J. Antibiotics, 33, 665-667), neothramycins A and B (Takeuchi et al., 1976 J. Antibiotics, 29, 93-96), porothramycin (Tsunakawa et al., 1988 J. Antibiotics, 41, 1366-1373), prothracarcin (Shimizu et al., 1982 J. Antibiotics, 35, 972-978; Langley and Thurston, 1987 J. Org. Chem., 52, 91-97), sibanomicin (DC-102)(Hara et al., 1988 J. Antibiotics, 41, 702-704; Itoh et al., 1988 J. Antibiotics, 41, 1281-1284), sibiromycin (Leber et al., 1988 J. Am. Chem. Soc., 110, 2992-2993) and tomamycin (Arima et al., 1972 J. Antibiotics, 25, 437-444).
PBDs are of the general structure:
They differ in the number, type and position of substituents, in both their aromatic A rings and pyrrolo C rings, and in the degree of saturation of the C ring. There is either an imine (N═C), carbinolamine (NH—CH(OH)) or a carbinolamine methyl ether (NH—CH(OMe)) at the N10-C11 position which is the electrophilic centre responsible for alkylating DNA. These forms may exist in equilibrium in solution. All of the known natural products have an (S)-configuration at the chiral C11a position which provides them with a right-handed twist when viewed from the C ring towards the A ring. This gives them the appropriate three-dimensional shape for isohelicity with the minor groove of B-form DNA, leading to a snug fit at the binding site (Kohn, 1975 In Antibiotics III. Springer-Verlag, New York, pp. 3-11; Hurley and Needham-VanDevanter, 1986 Acc. Chem. Res., 19, 230-237). Their ability to form an adduct in the minor groove enables them to interfere with DNA processing, hence their use as antitumour agents.
The present inventors have previously disclosed that PBDs can be employed as prodrugs by protecting them at the N10 position with a nitrogen protecting group which is removable in vivo (WO 00/12507). Many of these protecting groups are carbamates, and are, for example, of the structure:
where * indicates the attachment point to the N10 atom of the PBD. These protecting groups are described as being added to the compound at two different stages in the synthesis route. One stage is addition of the corresponding chloroformate to a precursor structure as follows, which precursor is then cyclised to form the desired final compound:

An alternative route discussed involves adding the corresponding chloroformate to a carboxy aniline precursor of the starting material in the above route.
These routes employ a chloroformate reacting with an (aromatic) amine to form the carbamate. The present inventors have discovered that the use of the chloroformate has disadvantages, and have therefore developed an alternative synthesis route, which does not employ a chloroformate.